An alternative form of the cohort study is something termed the retrospective cohort study.  Other researchers may also call this a historical prospective study.  This design is nearly identical to the prospective cohort study.  The sequence of baseline exposure determination and longitudinal follow-up for outcomes is similar.  The difference lies in the time in which the study begins.  In this retrospective design, the researcher constructs the cohort study by looking back in time and placing data in the appropriate order and sequence.  These studies are possible to do with large medical databases, such as the membership files of the Health Maintenance Organizations, or the medical files in the Scandinavian countries.